Current gas turbine engines continue to improve emissions and engine efficiencies. Notwithstanding these improvements, further increases in engine efficiencies will require more effective use of a mass of compressed air exiting a compressor. Gas turbine engines normally use the mass of compressed air for: 1) combustion air, 2) dilution air, 3) combustor cooling air, and 4) turbine component cooling air. Each use of the mass of compressed air may vary according to a load on the gas turbine engine. Generally each of these uses requires more of the mass of compressed air as the load increases.
In particular, combustion air and combustor cooling air have increased in importance with increasing regulations of NOx (an uncertain mixture of oxides of nitrogen). The efficiencies of the gas turbine engine usually improve with increased temperatures entering a turbine. Unlike the efficiency of the gas turbine engine, decreasing NOx production in gas turbine engines typically involves reducing a flame temperature. Lean premixed combustion attempts to decrease NOx production while maintaining gas turbine engine efficiencies. A lean premixed combustor premixes a mass of combustion air and a quantity of fuel upstream of a primary combustion zone. Increasing the mass of combustion air reduces the flame temperature by slowing a chemical reaction between the fuel and the combustion air. By reducing the flame temperature, NOx production also decreases.
Even with the lower flame temperatures, a liner wall of the combustor must be maintained at an operating temperature meeting a durability requirement. A number of cooling schemes may be used to cool the combustor liner including film cooling, convection cooling, effusion cooling, and impingement cooling. However, film cooling often times results in an increase in carbon monoxide (CO) production. Instead, many manufactures currently rely on backside cooling of combustor liners to reduce the production of CO.
At low engine loads, decreasing flame temperatures reduce requirements for cooling air and combustion air. The lower flame temperatures nonetheless lead to increased CO production and lower flame stability. Designing for both the high load and low load engine conditions generally results in very complex solutions. Typical designs focus on controlling the mass of combustion air to an individual injector. These controls require tight tolerances on dimensions of the injectors. Even with injectors having tight tolerances, the actuation of the injectors must be equally precise to avoid a mal distribution of combustion air entering the injectors.
The present invention is directed at overcoming one or more of the problems set forth above.